This invention relates to a lost foam metal casting process that comprises independently and concurrently casting two metal alloys to form an integral body. More particularly, this invention relates to a lost foam engine block casting comprising distinct sections formed of different aluminum alloys that are concurrently cast to fuse the alloys into an integral product.
In a typical lost foam casting process, a pattern is formed of a polymeric material vaporizable at metal casting temperatures, such as molded expanded polystyrene. The pattern is embedded in an unbonded sand body to form a foundry mold. Molten metal is cast into the mold in contact with the pattern, whereupon the metal progressively decomposes and replaces the pattern. In this manner, the pattern is duplicated by the metal, whereafter the metal solidifies to form a product casting.
A common internal combustion engine of the type employed to power automotive vehicles comprises an engine block formed of a relatively massive and complex metal casting. Depending upon the particular engine design, the engine block casting comprises one or more walls that define a desired number of cylinder bores. During engine operation, a piston reciprocates within each bore to drive a common crankshaft. The crankshaft is housed within a crankcase formed at least in part by a section of the engine block casting. Also, the engine block casting comprises an outer wall located about the cylinder walls, but spaced apart therefrom to define a water jacket through which coolant is circulated during engine operation. Thus, the engine block is formed of a single casting that comprises a cylinder wall section, a crankcase section and a water jacket wall section.
Although historically most engine blocks have been formed of cast iron, engine blocks have also been formed of aluminum castings. Aluminum blocks have been cast by empty-cavity processes such as die casting or permanent mold casting. It has also been proposed to cast aluminum blocks by a lost form process. Aluminum alloys of the type used in engine blocks have properties dependent upon the silicon content and are described by reference to a low melting eutectic composition formed between aluminum and about 12 percent silicon. Hypereutectic alloys contain silicon in an amount greater than the eutectic and are characterized by the presence of free silicon particles dispersed in a eutectic matrix. The free silicon particles form a hard phase that improves wear resistance. This wear resistance is desired in the bore wall to reduce scuffing by the piston. However, hypereutectic alloys are more difficult to cast, particularly in a large body such as an engine block. Early precipitation of the silicon particles hinders the flow of feed metal into slow solidifying, interior regions of the casting, resulting in increased macroporosity. Also, hypereutectic alloys exhibit increased microporosity attributed to added heat released during silicon precipitation and increased thermal contraction during cooling. Although desired for cylinder walls, the hypereutectic alloy renders the casting more difficult to machine, as evidenced by increased tool wear, and is not considered advantageous in sections remote from the cylinder wall. Of particular significance to this invention, the casting of hypereutectic alloy by a lost foam process presents difficulties peculiar to that process. Loss of heat at the melt front adjacent the decomposing pattern results in premature solidification, as evidenced by cold fold defects in the product casting.
On the other hand, hypoeutectic alloys, which contain less silicon than the eutectic, are characterized by dispersed aluminum particles that are soft in contrast to the hard silicon particles in the hypereutectic alloy. This facilitates machining of the casting, but also increases wear at the bore wall. It is known to insert a tubular liner formed of wear-resistant alloys, such as cast iron or hypereutectic aluminum alloy, into cylinder bores in the hypoeutectic casting to provide a satisfactory piston contact surface. Liners require precision machining of both inner diameter and outer diameter surfaces and careful assembling into the engine block to obtain a proper fit, thereby adding significantly to the cost of the engine. Also, depending upon engine design, stresses created by differential thermal expansion between the liner and the surrounding cylinder wall may result in crack formation during engine operation.
For these reasons, engine block castings formed solely of hypereutectic alloy or formed solely of hypoeutectic alloy have not been entirely satisfactory.
It is an object of this invention to provide an integral casting formed by a lost foam process and comprising a first section formed of a first alloy and an independently but concurrently cast second section formed of a second alloy compositionally distinct from the first alloy.
It is a more particular object of this invention to provide a compound aluminum alloy engine block casting comprising a cylinder bore wall section formed of a first alloy, preferably hypereutectic aluminum alloy, and a remainder formed of a second alloy compositionally distinct from the first alloy, preferably hypoeutectic aluminum alloy.
It is also an object of this invention to provide a lost foam process for casting metal which comprises providing an expendable pattern having portions corresponding to sections in the product, and casting a first alloy to replace a first portion of the pattern and a compositionally distinct second alloy to replace a second portion of the pattern, such that the resulting integral product comprises a first section formed of the first alloy and a second section formed of the second alloy.
It is a more particular object of this invention to provide a lost foam casting process for producing an engine block casting from a pattern sized and shaped corresponding to the desired engine block configuration, which process comprises casting a melt of a hypereutectic alloy to duplicate a portion of the pattern corresponding to an engine wall section of the engine block casting and independently casting a hypoeutectic alloy to replace portions of the pattern corresponding to the crankcase section and the water jacket wall section.